<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    html,
    body {
      height: 100%;
    }

    .header {
      height: 50%;
      width: 50%;
      background-color: pink;
      margin: auto;
    }

    .main {
      width: 100%;
      height: 50%;
      /* padding-top:50%; */
      background-color: #ddd;
    }

    .left,
    .right {
      float: left;
      /* padding-top:100%; */
      height: 100%;
      width: 50%;
      background-color: green;
    }

    .right {
      background-color: blue;
    }
  </style>
</head>

<body>
  <div class="header"></div>
  <div class="main">
    <div class="left"></div>
    <div class="right"></div>
  </div>
</body>

</html>